Precision Lattice-Parameter Determination of (Mg,Fe)SiO3 Tetragonal Garnets

Science  09 Feb 1990:
Vol. 247, Issue 4943, pp. 697-699
DOI: 10.1126/science.247.4943.697


The tetragonal garnet (Mg,Fe)SiO3 is a high-pressure phase of pyroxene that is thought to be a major constituent of the earth's upper mantle. Its crystal structure is similar to that of cubic garnet, but it is slightly distorted to tetragonal symmetry so that its x-ray powder diffraction pattern shows a very small line splitting. A suite of tetragonal garnets with different compositions in the MgSiO3-rich portion of the MgSiO3-FeSiO3 system was synthesized at about 20 gigapascals and 2000°C. The lattice parameters a and c of quenched samples were determined by whole-powder-pattern decomposition analysis of Fe Kα x-ray powder diffraction data, which has the capacity to resolve to a high degree heavily overlapping reflections. It was found that the lattice parameters can be obtained from the following equations; a (in angstroms) = 11.516 + 0.088x and c (in angstroms) = 11.428 + 0.157x, where x, teh mole fraction of FeSiO3, is 0.0 ≤ x ≤ 0.2.